blob-polyfill => fetch-blob and use /tmp for LocalStorage

[jq170727]

This request proposes to fix two small problems in userbase-js-node:

• "blob-polyfill" no longer works, generating instead the following error:

  % node index.js 
    xxx/node_modules/blob-polyfill/Blob.js:519
                              "-ms-scroll-limit" in document.documentElement.style &&
                                                    ^
    ReferenceError: document is not defined
  

• On Vercel the current directory isn't writable by server-less functions.

To address these issues it:

• replaces "blob-polyfill" with the newer "fetch-blob"
• moves the parent of LocalStorage to "/tmp"

These changes work for my userbase use-case but would appreciate feedback and/or
suggestions as to alternative solutions.